Add FileAPI endpoints

[Foorack]

Fixes #6

The comment on the Docsify/Markdown version was right, the FileAPI and especially File upload is indeed wonky. Sometimes requests just times out with no response. As the SDK is the only part uploading files through these endpoints there is also incredibly limited information/documentation on any of these. The Gallery/Icon upload on the website uses completely different endpoints.

This FileAPI documentation ideally needs to be improved even further in future updates. It would be good to add more instructions and an outline of the whole timeline from start to finish with all the required steps to upload a file with delta and signature. More testing is also needed to see if different edge cases are covered (e.g. simple vs multipart upload).

[Foorack]

This should cover the base implementation of uploading, managing, and deleting files.

Further documentation, especially in areas to how .signature and deltas are calculated and generated, as well as a complete overview picture of the flow from start to finish is severely needed, but something which should be added in a future commit. The cost of perfection is infinite; this is a starting point, and right now there are more critical areas to cover such as Avatars, Instances and Worlds.

This will be revisited once automatic library generation has been further researched, and the various API calls can be automated. The FileAPI is particularly laborious to test manually.